I have figured out how to post images from Picasa. It seems Proboards won't embed an SSL/https image, just base http.
If you pull up an image in an album and click the 'Link to this Photo' on the right, you'll get two links generated.
The first is 'Link', and this link doesn't work. It is an https link and results in no image, even if you change the protocol to http (it just tries to redirect to the secure).
The second is 'Embed Image'. Choose the size you want and check the 'Image only (no link)'. This also generates an https-based link, but if you change this to http, it will result in an image (when placed in the img blocks of course).